1. Field of the Invention:
The field of the invention is in motor vehicle transmissions providing increased fuel energy utilization efficiency.
2. The Prior Art:
Conventional transmissions used in motor vehicles transmit torque from the engine to the wheels by two means: (1) the predominant torque transfer is "pass through", i.e., in response to the driver's command for more power, the flow rate of fuel is increased to the engine, the engine's torque is increased and the increased torque is "passed through" to the wheels (see Mode 1 for a typical engine torque map--line "X" shown in FIG. 1); and (2) when the driver's command for more power exceeds what the engine can supply at its initial speed, the driver must shift the transmission to a lower gear (either manually or by triggering the shift in an "automatic" transmission) to increase the engine's speed (see Mode 2--line "Y" in FIG. 1, for example). Since power to the wheels at a given vehicle speed is proportional to engine torque times engine speed, power to the wheels is increased by either increasing engine torque (fuel flow rate at a given engine speed) and/or by increasing engine speed. There are two principal disadvantages of such transmissions: (1) the engine is usually operating in Mode 1 and thus supplies torque at an average efficiency much less than the optimum available (e.g., in FIG. 1 point A represents an average value while point B represents the optimum available efficiency at that speed); and (2) when a change of gear is needed, there is an interruption in the supply of torque to the wheels, manifested as a "jerk". Automatic transmissions smooth this "jerk" through a torque converter; however, increased inefficiency is the result.
Much work has been devoted to replacing conventional transmissions and their inherent disadvantages. This work has focused largely on continuously variable transmissions (CVTs). Ideally, with a CVT an engine would operate along line "Z", the optimum efficiency line as shown on FIG. 1. CVT designs include is mechanical (e.g., variable ratio pulleys), electric (an electric generator driven by the engine "powers" an electric motor connected to the wheels--modern train locomotives utilize this design) and hydraulic which operates much like the electric design. These designs offer some improvements but still rely on Mode 1 (line "X" in FIG. 1--increased fuel rate at a given speed or, more generally when speed is changing, increased fuel rate per combustion event), as the means of increasing engine speed to increase power to the wheels. However, operation along the optimum torque curve shown as line "Z" in FIG. 1, leaves little remaining torque available above this optimum for rapidly increasing the speed of the engine (i.e., accelerating the rotating mass of the engine while overcoming increased friction) to quickly respond to the driver's command for increased power to the wheels. Rapid power response is a critical vehicle performance characteristic from a driver/customer's perspective.
There are two options currently recognized as improving the response of a CVT. The first option initially reduces the torque available to the wheels and applies this torque to accelerating the engine to the needed increased speed. However, this first option is commercially unacceptable because it results, not just in hesitation, but in an actual loss of power to the wheels, completely contrary to the driver's command for more. The second option reduces the standard operating curve downward from optimum so that more torque is available for Mode 1 function (See line "W" in FIG. 1), therefore resulting in a further efficiency trade-off while still not achieving the power response of conventional transmissions that can fully utilize both Mode 1 and Mode 2.